Currently, Compact Discs (CDs) have the largest market among various optical information recording mediums. CDs are recorded or reproduced with use of near-infrared semiconductor lasers whose wavelength is in a range of 780 nm to 820 nm. Digital Versatile Discs (DVDs) which have spread rapidly, are recorded or reproduced with use of red semiconductor lasers whose wavelength is even shorter and is in a range of 635 nm to 680 nm to make the light spot smaller.
From the market there is a demand for a drive device that can reproduce both the two types of optical information recording mediums conforming to different standards. Conventionally, a reproduction-dedicated optical pickup device, shown in FIG. 1, has been proposed (see, for example, Document 1). The operation principle of the conventional optical pickup device is as follows.
As shown in FIG. 1, an optical pickup device 2 includes light sources 3 and 4, a hologram element 7, a light receiving element substrate 14, and a reflection mirror 15, and reads information from an optical information recording medium 1. The light source 3 conforms to the DVD standard, and emits a laser light beam having 650 nm of wavelength. The light source 4 conforms to the CD standard, and emits a laser light beam having 780 nm of wavelength.
The reflection mirror 15 guides laser light beams emitted from the light sources 3 and 4 to the optical information recording medium 1. The hologram element 7 diffracts, by diffraction areas 5 and 6 thereof, laser light beams reflected from the optical information recording medium 1. The light receiving element substrate 14 includes light receiving elements 8-13, and receives light beams diffracted by the hologram element 7.
A laser light beam emitted from the light source 3 enters the light receiving elements 8-11. From output signals of the light receiving elements 8-11, a focus error signal is detected by the Spot Size Detection (SSD) method, a tracking error signal is detected by the Differential Phase Detection (DPD) method, and a reproduction signal is detected.
The laser light beam emitted from the light source 4 enters the light receiving elements 8, 9, 12, and 13. From output signals of the light receiving elements 8, 9, 12, and 13, a focus error signal is detected by the Spot Size Detection method, a tracking error signal is detected by the 3-beam method or the Push-Pull (PP) method, and a reproduction signal is detected.
It should be noted here that a write once read many CD (CD-R) can also be reproduced using the light source 3.
Document 1: Japanese Patent Publication No. 3518457